1. Field of the Invention
The present invention relates to an optical disk driving apparatus having an optical disk that is rotated according to a constant linear velocity (CLV) system and, more particularly, to an optical disk driving apparatus in which an optical pickup device can be translated on the optical disk at high speed and a method of translating the optical pickup device.
2. Description of the Prior Art
One recent optical disk driving apparatus tends to increase a recording density of an optical disk by rotating the optical disk at the constant linear velocity (hereinafter referred to as a CLV, if necessary). Further, recent optical disk apparatus tend to increase the transfer rate of data by increasing the revolution rate of an optical disk. U.S. Pat. Nos. 4,660,189 and 5,126,994 describe the above-mentioned optical disk driving apparatus of such as CLV system.
Generally, when an optical pickup device is translated along a radial direction of an optical disk (i.e., during a seek operation), the optical pickup device is translated under the focusing servo in order to record or reproduce a signal immediately after the seek operation. An upper limit of a revolution rate of the optical disk is determined by the limit frequency of the focusing servo, i.e., a frequency at which an open loop characteristic of the focusing servo crosses 0 dB. The limit frequency of the focusing servo is a limit frequency at which the optical pickup device can be translated by a motor in response to a focus error amount in order to effect the focusing servo.
An optical pickup device that is located at a relatively outer peripheral side of the optical disk is translated toward the inner peripheral side in about 0.1 second if the optical disk is a 12-inch write once optical disk. Also, it takes about 1.2 seconds for a optical disk to increase a rotational speed on the outer peripheral side, i.e., from a rotational speed of 1080 r.p.m. to a rotational speed on the inner peripheral side, i.e., 2160 r.p.m.
Therefore, when the optical pickup device is translated from the outer to inner peripheral sides of the optical disk, even if the optical pickup device is translated and the rotational speed of the optical disk is changed at the same time, then the rotational speed of the optical disk does not reach the rotational speed on the inner peripheral side track after the optical pickup device is translated toward the inner peripheral side. In this case, however, the focusing servo can follow such rotational speed because the frequency of the focusing servo becomes a value lower than a value desired by a limit frequency of the focusing servo.
When the optical pickup device is translated from the inner to the outer peripheral sides of the optical disk, if the optical pickup device is translated and the rotational speed of the optical disk is changed at the same time, then the rotational speed of the optical disk is not reduced sufficiently by the time the optical pickup device reaches the outer peripheral side of the optical disk. There is then the disadvantage that the frequency of the focusing servo exceeds the limit frequency of the focusing servo. As a consequence, the optical pickup device cannot follow the optical disk according to the focusing servo so that the optical pickup device is brought in contact with the surface of the optical disk.